This section provides background information related to the present disclosure which is not necessarily prior art.
The silicon controlled rectifier (SCR), which is a type of thyristor, is a widely used electronic component that can be easily controlled at start up, but its fast switch off detection is often difficult. Traditional ways of detecting the switch off detection of SCRs impose a number of limitations in performance in many applications where SCR are used, particularly in static transfer switches such as are used in UPS systems.
Typically, SCR switch off detection is mainly based on direct current analysis of the SCR. That is, the current flowing through the SCR is monitored and the SCR is determined to have switched off when the current flowing through it has dropped below a threshold, which is the holding current level of the SCR. However, SCRs having high amperage capacity in the thousands of amps can have a holding current level of only a few tens of milliamps. This presents difficulties and uncertainty when attempting to monitor the current flowing through the SCR and determining that this current has fallen below the holding current level of a few tens of milliamps. Considering a static transfer switch (STS) application, the sole direct current SCR switch off detection is not sufficient to guarantee a safe and fast transfer between the input sources. Consequently, the direct current analysis must be accompanied with other analysis (input voltage analysis, phase displacement analysis between voltage and current linked to the output load power factor, etc.). This additional analysis both requires significant additional CPU resources and results in delays in the transfer between sources depending on the particular working conditions of the STS.